There is something mysterious about Tony Blair just now. Hostility to him over the cash-for-peerages investigation, Iraq and much else does not abate. In interviews he's constantly asked when he's going to leave. The polls are terrible. He should be grey, worn down, despairing. Yet he seems almost perky. He ought to have given up on his "legacy" but he doesn't seem to have done. It is as if he knows something we don't.

Well, he does. There is a far-advanced, detailed plan for his life after Downing Street, which he hopes will keep him in the spotlight and save his reputation. It has been quietly worked on for 18 months. Key meetings this very week will take it forward. But what, you may ask, is so momentous that it has the faintest chance of blurring, if not eradicating, the appalling and bloody disaster that has been Iraq? What is bigger than that? Africa? Northern Ireland?

No, the answer is climate change. Blair has told friends he will embark on a mission to save the world from global warming. Some of those close to Blair have urged him to devote his time to earning huge sums of money making speeches and sitting on corporate boards. But he has decided instead to use his personal contacts, his reputation in America, his undoubted energy and his experience in compromise-broking to help bring world leaders to "Kyoto 2", the carbon emission treaty needed to replace the partial and deeply flawed first attempt, which runs out in five years' time. It will be "Tony saves the world".

The plan goes back to July 2005 at the Gleneagles G8 summit. There, amid the chaos of the London bombings and the grand declarations, Blair spotted a chance for a new diplomatic crusade. It was obviously necessary to bring the Americans on board, and friends say the prime minister had long realised this would be a post-Bush project. But just as important was gaining the support of the high-emitting fast-developing nations. The problem is well known: the US won't agree to anything that restricts its competitiveness while allowing the new economic powers such as China and India to leap ahead; while the latter insist that they must be allowed the same freedom to develop that the west enjoyed.

Blair saw an opportunity to broker a deal. A new group, Globe, was formed, bringing together parliamentarians from the G8 countries plus China, India, Brazil, South Africa and Mexico. According to Blair's friends, it was his idea. Its timetable is fairly urgent, since the replacement for Kyoto must be ready by 2010 to give a two-year run-up to implementation. Since a new US president won't be in place till 2009, it's clearly too late to wait until then to start negotiations - a potential deal needs to be ready and waiting for the new president ...